<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <canvas id="canvas" width="400" height="100"></canvas>
    <div class="text">恭喜您获得100w</div>
    <style>
      * {
        margin: 0;
        padding: 0;
      }
      .text {
        position: absolute;
        left: 130px;
        top: 35px;
        z-index: -1;
      }
    </style>
    <script>
      // js
      const canvas = document.getElementById("canvas");
      const ctx = canvas.getContext("2d");

      // 填充的颜色
      ctx.fillStyle = "darkgray";
      // 填充矩形 fillRect(起始X,起始Y,终点X,终点Y)
      ctx.fillRect(0, 0, 400, 100);
      ctx.fillStyle = "#fff";
      // 绘制填充文字
      ctx.fillText("刮刮卡", 180, 50);

      let isDraw = false;
      canvas.onmousedown = function () {
        isDraw = true;
      };
      canvas.onmousemove = function (e) {
        if (!isDraw) return;
        // 计算鼠标在canvas里的位置
        const x = e.pageX - canvas.offsetLeft;
        const y = e.pageY - canvas.offsetTop;
        // 设置globalCompositeOperation
        ctx.globalCompositeOperation = "destination-out";
        // 画圆
        ctx.arc(x, y, 10, 0, 2 * Math.PI);
        // 填充圆形
        ctx.fill();
      };
      canvas.onmouseup = function () {
        isDraw = false;
      };
    </script>
  </body>
</html>
